This invention relates to inserts for drag type rotary drill bits. More particularly, the invention relates to inserts comprising two components having an engaging element and a shank element which are connected together by brazing. A super abrasion resistant material is deposited upon one surface of the engaging element and this surface is used as the contact surface of the insert. A plurality of these anchored two component drill bit inserts are inserted into the crown of a drill bit.
2. Prior Art
The application of superhard material such as sintered polycrystalline diamond compacts to the contact portion or working surface of the engaging elements in rotary type drag bits is well known. The use of superhard material has extended the life expectancy of the drag bits, has allowed the design and development of rotary type drag bits with more agressive cutting actions and has improved penetration rates.
Several forms of drill bit inserts have been developed to take advantage of the properties of these super hard materials. One such structure of drill bit inserts involves a two component system comprising an engaging element and a shank. The engaging element in these inserts is a cemented carbide substrate in the shape of a cylindrical disc with a layer of polycrystalline diamond bonded to the substrate. The reverse surface of the engaging element is mounted onto the carbide shank which is then inserted into the crown of a drill bit. The cylindrical disc shaped carbide substrate is bonded to the carbide shank forming a flat-on-flat attachment area between the engaging portion and the shank portion. This prior art structure is shown in FIG. 1B. However, since the layer of super hard material, polycrystalline diamond for example, is physically close to the flat-on-flat bonding area and the diamond's crystalline structure degrades above certain temperatures the methods of forming the flat-on-flat bond are limited. The flat-on-flat bond can be accomplished by brazing provided the braze material becomes liquid below the degradation temperature limit of the polycrystalline diamond material. Thus, the type of braze material which can be used is limited. Other more complicated and expensive methods involving the use of heat sinks to protect the polycrystalline diamond material can also be used to form the flat-on-flat bond.
The two component insert is particularly useful since the length of the shank can be varied to provide the bit with a greater range of cutting depth and the rake or angle at which the engaging element contacts the rock can be varied by simply substituting inserts with engaging elements mounted to the shank at different angles rather than changing the structures on the crown. Nevertheless the prior art two component inserts with the flat-on-flat attachment between the engaging element and the shank has a number of defects. First, the engaging element, under the stresses encountered in drilling, can become unattached from the shank since the flat carbide surface of the engaging portion is generally brazed to a planar or flat surface on the shank. The angle of this planar interface between the engaging element and the shank to a vertical axis, for an optimum rake for cutting, is 10.degree. to 20.degree.. The flat-on-flat bond set at this particular angle is subjected to shear forces resulting from forces exerted on the engaging element from the bottom of the hole being drilled. These forces work to break the bond and separate the engaging element from the shank.
Second, the forces that are exerted on the cutting surface of the engaging element when it contacts new material are concentrated at the bottom-most portion of the engaging element. Due to the heterogeneous nature of rock, there is also a great deal of recurring side to side imbalance. This concentration of forces at the bottom most portion of the engaging element coupled with the imbalance of loads on the engaging element which occurs over the course of the drilling process weakens the flat-on-flat bond between the engaging element and the shank.
Third, with the prior art structure it is difficult to properly position the cylindrical disc shaped engaging element onto the shank. Since the positioning of the engaging element onto the shank requires a flat-on-flat bond, there is no way to assure that the engaging element is symmetrically positioned. Any even slight misalignment between the engaging element and the shank exaggerates the forces working to separate or break the flat-on-flat bond.
Fourth, if the flat-on-flat bond between the engaging element and the shank is formed by brazing, the braze material used must have a low melting point in order to prevent degradation of the crystalline structure of the polycrystalline diamond material as explained above. This limits the choice of brazing materials. When a braze material with a low melting temperature is used to prevent degradation of the polycrystalline diamond material during the bonding of the engaging element to the shank, the high temperatures created during the drilling operation cause this brazed bond to weaken thereby encouraging a break or fracture in the bond. The alternative methods of forming the flat-on-flat bond between the engaging element and the shank are expensive and time consuming which results in higher manufacturing cost for the inserts.